Compound Hydrometeorological Extremes: Drivers, Mechanisms and Methods

Abstract

Compound extremes pose immense challenges and hazards to communities, and this is particularly true for compound hydrometeorological extremes associated with deadly floods, surges, droughts, and heat waves. To mitigate and better adapt to compound hydrometeorological extremes, we need to better understand the state of knowledge of such extremes. Here we review the current advances in understanding compound hydrometeorological extremes: compound heat wave and drought (hot-dry), compound heat stress and extreme precipitation (hot-wet), cold-wet, cold-dry and compound flooding. We focus on the drivers of these extremes and methods used to investigate and quantify their associated risk. Overall, hot-dry compound extremes are tied to subtropical highs, blocking highs, atmospheric stagnation events, and planetary wave patterns, which are modulated by atmosphere-land feedbacks. Compared with hot-dry compound extremes, hot-wet events are less examined in the literature with most works focusing on case studies. The cold-wet compound events are commonly associated with snowfall and cold frontal systems. Although cold-dry events have been found to decrease, their underlying mechanisms require further investigation. Compound flooding encompasses storm surge and high rainfall, storm surge and sea level rise, storm surge and riverine flooding, and coastal and riverine flooding. Overall, there is a growing risk of compound flooding in the future due to changes in sea level rise, storm intensity, storm precipitation, and land-use-land-cover change. To understand processes and interactions underlying compound extremes, numerical models have been used to complement statistical modeling of the dependence between the components of compound extremes. While global climate models can simulate certain types of compound extremes, high-resolution regional models coupled with land and hydrological models are required to simulate the variability of compound extremes and to project changes in the risk of such extremes. In terms of statistical modeling of compound extremes, previous studies have used empirical approach, event coincidence analysis, multivariate distribution, the indicator approach, quantile regression and the Markov Chain method to understand the dependence, greatly advancing the state of science of compound extremes. Overall, the selection of methods depends on the type of compound extremes of interests and relevant variables.

Compound events and associated impacts in China

“…A variety of compound events have been evaluated in China, including compound hot extremes, compound dry-hot extremes, compound wet-hot extremes, and compound flooding, as shown in Figure 1 . These events are defined based on the combination of contributing variables of different time scales, which can be investigated by statistical approaches (e.g., empirical counting, indicator approach, and multivariate distribution), dynamical modeling approaches (e.g., climate model ensemble and hydrological model), and socio-physical approaches (e.g., event-based storylines) ( Bevacqua et al., 2021 ; Hao and Singh, 2020 ; Raymond et al., 2020 ; Zhang et al., 2021 ). Their impacts span multiple sectors, including ecosystems, water resources, agricultural production, human health, and infrastructure, which have been documented around the world.…”

“…Compound wet-hot extremes are usually defined as the consecutive or sequential occurrences of wet conditions (e.g., humidity, high precipitation, and flood) and hot extremes over a specified period. The combined impacts of high temperature and humidity on human discomfort or health have been well recognized ( Fischer and Knutti, 2013 ; Zhang et al., 2021 ; Zscheischler et al., 2020 ). Owing to space limitations, we focus on the wet conditions based on precipitation or flood in defining compound wet-hot extremes in the following sections.…”

Rising risks of compound extreme heat‐precipitation events in China

“…Extreme weather and climate events are often triggered by complex atmospheric physical processes and can cause hazards that contribute to severe socio‐economic risks (Zscheischler et al ., 2020; Zhang et al ., 2021). Compound extreme weather and climate events describe the combinations of two or more extremes that occur concurrently or sequentially (AghaKouchak et al ., 2014; Leonard et al ., 2014; Zscheischler et al ., 2020; Zhang et al ., 2021), which were first introduced by the Intergovernmental Panel on Climate Change (IPCC) Special Report on Climate Extremes (SREX) (IPCC, 2012). Compared with the individual extreme events, compound extremes often substantially amplify their harmful impacts, leading to more severe ecological and socio‐economic damages (Zscheischler et al ., 2014; 2018).…”

Recent Increase in the Occurrence of Snow Droughts Followed by Extreme Heatwaves in a Warmer World

“…Comparatively high temperature along with atmospheric moisture deficit of dry snow drought increases the evaporative demand of atmosphere (represented by VPD in this study), which in turn amplifies the warming (Chiang et al., 2018; Mukherjee & Mishra, 2021) and favors hot days by depleting SM and closing plant stomata (Lansu et al., 2020; Stegehuis et al., 2021). It should be noted that large‐scale atmospheric circulations, such as blocking highs, atmospheric stagnation, planetary wave, and subtropical highs, are also responsible for the occurrence of land‐based heatwaves (Zhang et al., 2021). These anomalies of atmospheric circulation could result from the internal variability of atmospheric circulation as well as the ocean conditions via teleconnections (Wehrli et al., 2019).…”